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FM1236 / FM1246 tuner required power supply quality?

Hello:

I am attempting to fix the quality of the video signal coming out of a similar tuner (it's on a PC TV card). Currently, the brighness of the picture is fluctuating on the high-brightness areas, in the rythm with the sound; the tuner is powered from a LM7805-type regulator (which I have already added to the card; intially there was none, and the interference on the picture was terrible).

I suspect the power supply quality.

The datasheets for the FM1236 / FM1246 specify the "peak-to-peak ripple voltage susceptibility" for the tuner section, but nothing for the IF section.

On the IF section power pin, the voltage is fluctuating, depending on the picture being received, with up to 6-8 mV pp over the TV frame, not counting the overshoots.

Before I go further and swap the regulator (LM1117 looks a good candidate, to improve the load regulation and overshoot) I would like to know more on the required power supply quality.

Would anybody have any reference information about that? Details of a reference design with these tuners? I had tested the Leadtek PVR2000 card and it seemed to have the same tuner as my card has, and it did not have this problem. If somebody had the tuner powering details for that card, that could help I think.

Thank you.

-- Andy

Reply to
Andy
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How about using a decent bypass cap (aluminum 'lytic)?

Reply to
Robert Baer

I read in sci.electronics.design that Andy wrote (in ) about 'FM1236 / FM1246 tuner required power supply quality?', on Sun, 9 Jan 2005:

The sound is FM, unless you also have baseband audio stuff with significant current draw fed from the same regulator, so it shouldn't cause amplitude modulation of the video. I doubt that the power supply is at fault.

--
Regards, John Woodgate, OOO - Own Opinions Only. 
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Reply to
John Woodgate

Yes, there is already a bypass on each of the two power pins: a couple of ceramics + 100uF Panasonic FC series.

Reply to
Andy

There are baseband audio outputs from the module, but it looks like indeed these "audio rhythm" fluctuations are not due to a problem with the power supply. I have now inserted a ground loop breaking device in the aerial input path, and these fluctuations are no longer there...

But there are sometimes luminosity artefacts such as luminosity alteration of a uniform background, around a contrasted object placed on that background, in the horizontal direction. These could still be due to an unsufficient power supply regulation (the supply current of the IF section of the module is 150mA max, that of the TV/FM tuner section -

120mA max).
Reply to
Andy

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